1. Field of the Invention
The present invention is directed to a method for producing computed tomography (CT) images of a body region periodically moving with resting and motion phases with a CT apparatus having an x-ray source moved around the body of the life form to be examined for producing the CT images, and is also directed to an apparatus for the implementation of such a method.
2. Description of the Prior Art
Subjects that move during the CT data registration often cause line artifacts in the reconstructed CT image or are presented unsharp with double contours. This problem particularly occurs given CT exposures of the heart or heart-proximate lung structures in CT systems having mechanical movement of the x-ray source, wherein the data registration time for an image lies on the order of magnitude of one heart cycle. The motion artifacts are then caused by the employment of data for image reconstruction that were registered during the rapid contraction phase of the heart. Such images can only be conditionally employed for medical diagnostic interpretive methods such as, for example, calcification screening or perfusion imaging.
CT images of the heart that are relatively low in motion artifacts can be acquired with electron beam computer tomography (EBT). The x-ray source can be move free of inertia by electromagnetic deflection of an electron beam. Significantly shorter data registration times and, thus, a reduction of the motion artifacts, thus can be achieved. The cost of an EBT system, however, are multiply higher than that of a convention CT apparatus. Moreover, the image quality obtainable with an EBTxe2x80x94apart from cardiac images low in motion artifactsxe2x80x94cannot compete with that of a conventional system.
Conventional CT systems of the third and fourth generations have a mechanical rotation of the x-ray source and currently achieve scan times below 1 second per 360xc2x0 revolution (full revolution). An adequately good image quality for heart exposures can be achieved with such systems when only the data measured during the resting phase of the heart are employed for the image reconstruction.
One possibility for achieving this demand is offered, for example, by the ECG-triggered CT exposure technique disclosed in European Application 0 370 341 and German OS 196 27 166. The R-waves of an ECG signal obtained while image data are being registered serve for triggering the data registration. The measurement of a sub-revolution or full revolution starts after recognition of a registered R-wave, namely with an empirically defined delay time relative to the R-wave. A shutoff of the radiation likewise triggered by the ECG does not ensue at the end of the resting phase of the heart.
Other methods disclosed, for example, in German PS 33 25 939 and U.S. Pat. No. 4,182,311 forgenerating heart images register the ECG signal during the measurement and employ only data from a specific heart phase for the image reconstruction insofar as possible. The determination of the desired region thereby ensues purely empirically according to standard values known from the literature. This is also true of what has become the standard ECG-triggered CT exposure technique, wherein the delay time is likewise only empirically determined. Such a method is disclosed in German PS 196 22 075. A disadvantage of the method with empirical definition of the delay time, or of the part of the heart cycle with respect whereto data are used, is the highly different correlation of the ECG signal and mechanical heart movement for various patients.
An object of the present invention is to provide a method and an apparatus of the type initially described wherein the risk of the occurrence of motion artifacts is diminished.
The above object is achieved in accordance with the principles of the present invention in a method and an apparatus for producing CT images of a body region which periodically moves with resting and motion phases, wherein an x-ray source focus is moved around the body of the subject under examination for registering data used for producing the CT images, and wherein a number of projections are registered, during at least one revolution of the x-ray source focus around the subject, preferably, and during a time duration that is at least equal to a cycle of the movement of the body region, and wherein the projection data are analyzed directly to determine whether the data were acquired during a resting phase or a motion phase, and wherein only those data are employed for image reconstruction that were found to be acquired during a resting phase.
The inventive method is thus an automatic method wherein the registered measured data are patient-specifically classified by analysis of the measured data themselves to determine whether they are usable, i.e. were acquired during a resting phase of the heart, or are unusable, i.e. were acquired during a motion phase, with only measured data acquired during a resting phase of the heart being utilized for the image reconstruction. The inventive method can be applied for arbitrary CT apparatuses of the third or fourth generations having one or more detector lines, namely for normal axial scans as well as for spiral scans. The projections classified as usable according to the inventive method can be employed for arbitrarily fashioned reconstruction methods.
In one version of the invention, for classification of the measured data, the ECG signal of the respective patient is utilized. The correlation of the ECG signal with the actual mechanical movement of the heart can, first, ensue with automatic or interactive evaluation of measured data and/or CT images of a reference examination, i.e. a number of test projections, and evaluation of the synchronously acquired ECG signal. In this way, the patient-specific delay between R-wave of the ECG signal and the trigger time of the radiator can also be quantitatively acquired for ECG-triggered CT exposures, leading to a significantly improved imaging and a significantly more efficient examination execution.
Thus, with the inventive apparatus, an operator has the possibility of interactively identifying specific images as being low in motion artifacts.